Photoconductive antennas (PCA) [1] and electro-optic (EO) crystals [2] are widely used as detectors for coherent measurement of THz fields. In both detection methods, a part, called the detection pulse, of the same ultrashort laser pulse that is used to generate the terahertz pulse is fed to the detector, where it arrives simultaneously with the terahertz pulse. The detector produces a different electrical signal depending on whether the detection pulse arrives when the electric field of the THz pulse is low or high. An optical delay line is used to vary the timing of the detection pulse.
Both methods, i.e. photoconductive detection and electro-optical sampling, suffer from a number of limitations. Indeed, in the case of photoconductive detection, it is well known that the lifetime of photo carriers in the semiconducting materials of a photoconductive detector limits the THz detection bandwidth. In the case of electro-optical sampling, the detection efficiency and bandwidth of electro-optic (EO) crystals is restricted by optical absorption at THz frequencies and by the phase mismatch associated to the optical rectification process underlying the detection process. Furthermore, THz fields of the order of few hundred kilovolts per centimeter can induce an over rotation of the probe polarization during EO sampling in mm length EO crystals, such as ZnTe for example, with a consequent distortion of the detected waveform.
It has been demonstrated that THz fields can be detected by nonlinear interaction in centrosymmetric media via the well-known electric-field-induced second-harmonic generation (EFISH) [3]. The same effect has been employed for probing millimeter-wave circuits [4] and for the measurement of liquid dynamics [5]. For THZ, it has been named terahertz-field-induced second harmonic generation or TFISH. Yet these measurements are not able to retrieve the amplitude and phase of the THz electric field and are hence incoherent.
More recently, TFISH has been employed as a tool for the coherent characterization of THz field. The beating of the TFISH signal with a local oscillator at the same frequency is used to map the THz electric field temporal trace. This was firstly observed for intense probe pulses generating supercontinuum at the TFISH wavelength [6].
A further optimization of this scheme, named Air Biased Coherent Detection (ABCD), relies on an external DC field to bias the THz-probe pulse interaction region, thus providing the required local oscillator signal [7]. Further developments have been proposed for air-based detection of ultra-broadband THz pulses via electric field induced second harmonic generation, such as direct injection of the local oscillator field [8] or the analysis of the THz-probe spectrogram.
The Air Biased Coherent Detection (ABCD) method, based on the Terahertz Field Induced Second Harmonics (TFISH) process occurring in ambient air (or selected gasses), allows recording Terahertz fields with extremely large bandwidth, i.e. above 20 THz, depending on the duration of the sampling pulse. However, miniaturization and integration remains a main concern. Indeed the existing systems cannot be easily miniaturized to the extent of a few centimeters. Furthermore these detection systems require the use of high voltage amplifiers, typically kV sources, thus limiting the use of the method. Moreover, since the detection occurs in air, the use of high voltage is limited by the air breakdown voltage.
The current systems for the Air Biased Coherent Detection (ABCD) method are composed of a pair of electrodes, suspended in air and typically separated by a distance of 1 millimeter, which corresponds to a region where THz and optical beams are focused. In this region, an external ac bias electrical field of about 2 KV, 500 Hz is typically applied to the optical focus in order to generate a bias optical field. The higher the external bias, the better the signal. The external ac bias electric field oscillates at a repetition rate that is half of the laser repetition rate. As the detection signal to noise ratio (SNR) depends on the amplitude of the bias field applied, high bias fields are necessary to obtain a good signal. Since the detection occurs in air, a first limitation is thus the air breakdown voltage (3·106 V·m−1), which is the maximum bias field that can be applied between the electrodes before the appearance of corona discharges leading to the breakdown of the system. Moreover, the method is also limited by the distance between the electrodes, which cannot be reduced for mechanical reasons. Furthermore, due to the low nonlinear coefficient of air, intense probe pulses are required, which can only be delivered by amplified systems, such as CPA (chirped pulse amplification) for example. These laser systems are expensive and feature large footprints, which considerably limits the accessibility of the method.
Thus there is still a need in the art for a method and system for a fully-coherent terahertz detection.